collectionView reloadSections导致的

页面层级结构：

VC.View -> scrollView -> HeaderView -> SliderView -> CollectionView -> Cell -> RankRadioIconView -> (label、image)

Masnory警告：

(
    "<MASLayoutConstraint:0x283e65980 RankRadioIconView:0x10aa0a100.left == UIView:0x10aa0a4a0.left>",
    "<MASLayoutConstraint:0x283e65b60 RankRadioIconView:0x10aa0a100.right == UIView:0x10aa0a4a0.right>",
    "<MASLayoutConstraint:0x283e65f20 SDAnimatedImageView:0x10aa05f10.left == RankRadioIconView:0x10aa0a100.left>",
    "<MASLayoutConstraint:0x283e65ce0 SDAnimatedImageView:0x10aa05f10.right == RankRadioIconView:0x10aa0a100.right>",
    "<MASLayoutConstraint:0x283e66520 UILabel:0x10aa0bfa0.left == SDAnimatedImageView:0x10aa05f10.left + 10>",
    "<MASLayoutConstraint:0x283e665e0 UILabel:0x10aa0bfa0.right <= SDAnimatedImageView:0x10aa05f10.right>",
    "<NSAutoresizingMaskLayoutConstraint:0x283952030 UIView:0x10aa0a4a0.width == 0>"
)

scrollView、HeaderView均使用frame约束，SliderView、CollectionView、Cell、RankRadioIconView、(label、image)都是用Masonry约束，UIView:0x10aa0a4a0.width == 0该view为Cell的contentView。起初怀疑是frame、Masonry一同使用姿势不对导致的，排查半天没发现异常使用。最终在看到：

[self.collectionView reloadSections:[NSIndexSet indexSetWithIndex:0]];

在VC的ViewDidLoad中用frame布局完scrollView、HeaderView后传入collectionView数据源调用了该行代码，换成[self.collectionView reloadData];后就没有警告了。

思考

reloadData和reloadSections的刷新方式肯定不同，在reloadData后的 collectionView:cellForItemAtIndexPath中cell.frame不为0，而reloadSections后cell.frame为0，导致Masonry告警。（cell采用注册方式创建[collectionView registerClass:[Cell class] forCellWithReuseIdentifier:@"CellID"];）